A portfolio is a living, breathing, curated document (traditionally website) that demonstrates the skills, process, and potential of a designer / researcher / content strategist, etc.
You can create your portfolio on whatever platform you desire. Many designers create a website, use Behance, present via slide deck, or even use platforms like Notion or PDF formats. As long as you can clearly explain your process and showcase your projects effectively, choose whatever you feel most comfortable with. I highly recommend using a website builder like Webflow, Framer, Squarespace, or even free options like GitHub Pages, as they give you more control over presentation and make it easy for employers to access and share your work.